Microelectronic elements are typically packaged and assembled with a microelectronic component to facilitate connection to external circuitry. Heat is generated in use, as well as during manufacturing operations such as, for example, bonding. When heat is generated within the assembly, the various parts of the assembly expand and contract according to the coefficient of thermal expansion for the particular part. Incorporating various materials having different coefficients of thermal expansion can create stress on certain components within an assembly.
For example, as disclosed in certain embodiments of U.S. Pat. No. 5,518,964, the disclosure of which is hereby incorporated by reference herein, leads extend between a semiconductor chip and another component and are connected to contacts of the semiconductor chip by bonding material. For example, the semiconductor chip is assembled with a connection component, which may incorporate a dielectric body and leads extending on a lower surface of the dielectric body. The leads have a first end that is connected to the contacts and a second end connected to the dielectric body. A dielectric layer is formed around the leads and around the connection between the leads and the contacts. For example, a curable material is introduced between the chip and the dielectric body. The material is cured to form a dielectric layer surrounding the leads. On the upper surface of the dielectric body, the surface facing away from the leads, the dielectric body has terminals for forming connections with other components. For example, the terminals may ultimately be used to connect to conductive features on an external element, such as a circuit board.
During service, or during any operation in which heat is generated, some materials within the assembly have significantly different coefficients of thermal expansion from other materials in the assembly so that some parts expand and contract by different amounts from other parts of the assembly. The dielectric body of the component may comprise polyimide and the semiconductor chip may comprise silicon. These materials have coefficients of thermal expansion that are significantly different, which means that these parts of the assembly experience differing amounts of expansion and contraction for the same temperature change. The dielectric layer and the leads provide the assembly with flexibility so that the terminals move relative to the contacts on the chip. The dielectric layer and leads compensate for different dimensional changes within the assembly. The larger the dielectric layer in the vertical direction, the more movable the terminals and contacts are with respect to one another in the horizontal direction. However, the larger the dielectric layer, the more stress the leads experience. This is compounded by the presence of bonding material between the contacts and the leads, which interferes with the flexibility of the dielectric layer. For connections that do not add to the vertical height between the lead and the contact, the foregoing effect is minimal. However, some connections incorporate a significant amount of bonding material, which adds to the height of the connection between the leads and contacts, and impacts the reliability of the assembly.
Improvements to reduce stress on the conductive elements of a microelectronic assembly and improve the reliability of such assemblies are desirable.